The phenolic resin provides a molding excellent in mechanical properties, and hence has widely been employed from of old either independently or in the form of a blend with another resin, such as an epoxy resin. However, the phenolic resin per se and blend have drawbacks in that their light and alkali resistance is relatively low, that they are likely to absorb water or an alcohol to thereby suffer from changes in the dimension and electrical resistance, and that the thermal resistance, especially the oxidation resistance at high temperatures, thereof is poor.
In order to overcome the drawbacks, various modifications of the phenolic resin have been studied. For example, various modified phenolic resins have been proposed, which have improved resistance to deterioration and oxidation due to light, chemicals, etc. by virtue of the modification using a fat, an oil, a rosin or a neutral aromatic compound.
For example, Japanese Patent Laid-Open Publication No. 61(1986)-235413 discloses a phenolic resin having excellent thermal resistance, obtained by selecting reactants of a phenol-modified aromatic hydrocarbon resin. However, the phenolic resin obtained by this method is disadvantageously not cured unless being maintained at a high temperature for a prolonged period of time in the manufacturing of a molding by the use thereof.
Japanese Patent Laid-Open Publication No. 2(1990)-274714 discloses that a modified phenolic resin useful for a molding material, having excellent thermal and oxidation resistance and mechanical strength as cannot be expected from the conventional phenolic resin, is obtained by employing a petroleum heavy oil or pitch, which is a cheap material, as a modifier material and by selecting specific reaction conditions.
Further, Japanese Patent Laid-Open Publication No. 4(1992)-145116 discloses that, in the production of such a phenolic resin, a crude modified phenolic resin obtained by a polycondensation of starting compounds is subjected to a neutralization treatment, a water washing treatment and/or an extraction treatment to thereby neutralize and remove any acid remaining in the crude modified phenolic resin, so that a modified phenolic resin which does not corrode a metal member brought into contact with the resin is provided.
In the above process for producing the modified phenolic resin, the acid remaining in the crude modified phenolic resin is actually neutralized and removed by the neutralization treatment using an amine, followed by the water washing treatment. However, the modified phenolic resin obtained through the purification step involving the above neutralization and water washing treatments is likely to retain a neutralization product therein, so that there is a problem that it is unsatisfactory as a molding material used for a product on which strict requirements for thermal and corrosion resistance are imposed, such as a molding material for electrical or electronic part and a material for semiconductor sealer.
Japanese Patent Laid-Open Publication No. 6(1994)-228157 teaches that a modified phenolic resin containing substantially no acid can be obtained by purifying a crude modified phenolic resin through a purification step including a specific extraction treatment. The modified phenolic resin containing substantially no acid, obtained through this purification step, may be combined with an epoxy resin, so that a molding material can be obtained, which not only has excellent thermal and moisture resistance but also does not corrode any metals.
However, the above modified phenolic resin has a drawback in that the melt viscosity of the resin is so high that the resin is not suitable for speedy mass production of a molded article having a complex configuration. In addition, further improvements of thermal resistance, dimensional stability and strength and other mechanical properties have been demanded in the use of the modified phenolic resin in combination with an epoxy resin.
The present inventors proposed a process for producing a highly reactive modified phenolic resin having a low resin melt viscosity and an improved reactivity with the epoxy resins by means of reacting a modified phenolic resin with phenols in the presence of an acid catalyst to thereby lower the molecular weight of the modified phenolic resin (Japanese Patent Laid-Open Publication No. 7(1995)-252339).
In the molecular weight lowering step of this process, it is considered that the acetal bonding and/or methylene ether bonding present in the molecule of the modified phenolic resin is broken and dissociated to thereby lower the molecular weight of the modified phenolic resin and that phenols are bonded to dissociation terminals to increase a phenol content. Therefore, the molecular weight lowering reaction is typically carried out at a temperature at which the acetal bonding and/or methylene ether bonding in the modified phenolic resin molecules are cleaved and dissociated, i.e., 50-120.degree. C.
The highly reactive modified phenolic resins obtained as described above are relatively low in viscosity and are capable of providing a molding material having good thermal resistance and moldability, as well as superior mechanical strength such as dimensional stability when combined with an epoxy resin.
However, the viscosity of the highly reactive modified phenolic resin obtained in the process described above is not sufficiently low though it is significantly lower than those of the conventional modified phenolic resins. Especially, in the application for semiconductor sealers, there has been a demand for a lower viscosity to further improve the moldability while maintaining a high reactivity with the epoxy resin.
In this connection, resin molding materials tend to be expanded when they absorb moisture to deteriorate the dimensional stability. When the resin molding material is used for a composite material with a metal such as resin portions of electrical or electronic parts and, in particular, for the semiconductor sealers, the moisture adsorbed by the resultant resin package rapidly vaporizes during solder mounting at a high temperature. This causes swelling and cracks of the resin package. The resin portions containing moisture may corrode a metal portion, significantly affecting lifetime and reliability of a resultant product. It has thus been desired to reduce moisture absorption of the molding material comprising the highly reactive modified phenolic resin described above, for the applications where the moisture absorption of the molding material is undesirable.
The inventors have made extensive and intensive studies with respect to such drawbacks associated with the prior art. As a result, it has been found that a modified phenolic resin having a lower viscosity that cannot be obtained by a conventional molecular weight lowering reaction is produced while maintaining a high reactivity with epoxy resins by means of reacting a reaction product of polycondensation reaction as it is or after subjecting to purification with a phenol at a certain temperature condition in the absence of a formaldehyde polymer and other cross-linking agents and in the presence of an acid catalyst to lower the molecular weight thereof, and that a molding material formed of a combination of this modified phenolic resin and an epoxy resin has a lower moisture absorption. The present invention was thus completed.